1. Field of the Invention
The present invention relates generally to a method for achieving the highest vacuum possible in the least amount of time for a given vacuum chamber. More specifically, the present invention relates to a method of performing a bakeout on a vacuum chamber involving cycle purging the chamber.
2. Background of the Related Art
Vacuum chambers are well known. Vacuum chambers can be used to manufacture integrated circuits (ICs) on silicon wafers. Physical vapor deposition (PVD) chambers are one type of vacuum chamber for manufacturing ICs. PVD chambers usually must reach about 6.0.times.10.sup.-9 torr to qualify for use in manufacturing ICs.
The traditional method used for qualifying the vacuum of an ultra high vacuum chamber is called a bakeout. The bakeout process accelerates the removal of contaminants from the chamber, including driving out water vapor and other gases from the chamber components. The bakeout process is used to determine the highest vacuum, or lowest pressure, that the vacuum chamber can attain. A vacuum chamber's highest vacuum may be limited by a variety of reasons, including leaks in the chamber or contaminants in the chamber.
Various gases that have adsorbed onto the walls, instruments or other interior surfaces of the chamber may comprise the contaminants that limit a chamber's highest vacuum potential. When a chamber is subjected to a vacuum, these gases come out of the interior surfaces of the chamber in a process called degassing, or outgassing or desorption. When the pressure is returned to atmospheric pressure, such as prior to opening the chamber, water vapor and other gases in the chamber may be adsorbed into the interior surfaces of the chamber, only to be outgassed later when the vacuum returns.
The first time that a vacuum chamber is subjected to a vacuum, i.e., during the first bakeout, there may be a considerable amount of contaminants, absorbed or adsorbed gases, that must be outgassed. Subsequent times that the vacuum chamber is subjected to a vacuum should not have as many contaminants to outgas if the chamber can be kept relatively free of such contaminants in the interim. Thus, the first bakeout process is usually the longest period during which outgassing must be done.
The conventional method used to obtain an ultra-high vacuum for a chamber is to pump the chamber using a roughing pump to about 100 millitorr, then crossover to the high vacuum pump. After the chamber reaches a threshold pressure, an extensive bakeout, e.g. 36 hours in a PVD chamber, is performed. During the bakeout process, the high vacuum pump continuously pumps the chamber. Afterwards, the chamber must be cooled to ambient temperature in order to reach the base pressure, which may take about ten more hours.
An example of the conventional bakeout method can be seen in FIG. 1, which is a graph of pressure (in torr) inside the vacuum chamber versus time (in hours). Prior to time 100, the roughing pump brings the vacuum down to about 0.1 torr. Then the high vacuum pump, e.g., cryogenic pump, reduces the vacuum as low as about 2.times.10.sup.-6 torr. At time 100, the bakeout lamps are turned on and the chamber will heat up, causing extra outgassing in the chamber. This extra outgassing causes a rise in the pressure immediately after time 100 for 3-4 hours, after which, the pressure slowly drops for the next 10-15 hours as fewer and fewer contaminants remain to be outgassed from the chamber. At time 102, if the chamber has a wafer bakeout heater, the heater may be turned on, thus causing another rise in the pressure after time 102 when more outgassing from the heater occurs. At time 104, the bakeout test is done, the lamps and heater are turned off, and the chamber is permitted to cool until the final qualifying pressure may be determined. The time to perform this bakeout test is about 36 hours for a PVD chamber.
The problem that slows down the conventional method of performing the bakeout is the rise in pressure just after the bakeout lamps are turned on. This pressure rise is due to the speed at which the gases desorb from the chamber and are removed. For example, the outgassing rate can be calculated from the outgassing rate per area (typically 5.0.times.10.sup.-7 torr.multidot.liter/s.multidot.cm.sup.2) and the total interior surface area (typically 1.0.times.10.sup.+4 cm.sup.2 for a PVD chamber). The product of these two parameters gives a typical outgassing rate of 5.0.times.10.sup.-3 torr.multidot.liter/s. By comparison, the cryopump's throughput can be calculated from its effective pumping speed (typically 300 to 500 liter/s with the cryopump restrictor) and the pressure in the chamber (about 5.0.times.10.sup.-6 torr to 2.0.times.10.sup.-6 torr). The product of these two parameters is the typical throughput for a cryopump: between about 1.5.times.10.sup.-3 torr.multidot.liter/s and about 1.0.times.10.sup.-3 torr.multidot.liter/s. Thus, contaminants are outgassing almost five times as fast as the cryopump can remove them, so the pressure goes back up almost an order of magnitude after initially being reduced at time 100.
The difference between the outgassing rate and the pump throughput indicates a very inefficient procedure that is permitting part of the desorbed contaminants to re-adsorb onto the interior surface of the chamber. The typical gas composition for a typical high vacuum chamber with an O-ring seal may contain as much as 50% water vapor, so water vapor and other contaminants are continually re-adsorbing onto the chamber interior, causing a long slow bakeout process.
For the process described by FIG. 1, the increase in pressure when the lamps are turned on at time 100 is known to have two parts to it. The pressure is related to temperature according to the general gas law: EQU P=nKT, Equation 1
where P is pressure, n is the number density of the gas molecules, K is Boltzman's constant, and T is temperature. The change in pressure may be due to a change in either the temperature T or the density n. Therefore, for a change in pressure, the general gas law may be expressed as: EQU .DELTA.P=nK.DELTA.T+KT.DELTA.n. Equation 2
If Equation 2 is divided by Equation 1, the result is: EQU .DELTA.P/P=.DELTA.T/T+.DELTA.n/n. Equation 3
The values for .DELTA.P/P and .DELTA.T/T can be calculated from the readings from the pressure measurement 28 and the temperature measurement 30 (see FIG. 3). A typical value for .DELTA.P/P during bakeout is in the range of 5-7, while a typical value for .DELTA.T/T is less than 1. From Equation 3, a typical value for .DELTA.n/n is in the range of 4-6, which is greater than the range of values for .DELTA.T/T. Therefore, a substantial amount of the cause for the rise in pressure after time 100 in the conventional bakeout process is due to the change in density of the gas, which confirms that the cryopump is not able to pump out the desorbed gases fast enough to keep up with the outgassing rate.
Another method of performing a bakeout is shown in U.S. Pat. No. 5,536,330, issued Jul. 16, 1996, assigned to Applied Materials, Inc. of Santa Clara, Calif., and incorporated herein by reference as if fully set forth below, and assigned in common with the present application. This patent describes a bakeout process that starts with preheated gas sweeping the vacuum chamber. While maintaining a low vacuum, about 50 to 750 torr, hot gas is flowed through the chamber to sweep out as much of the degassing molecules as possible. Then the vacuum is pumped down to an ultra high vacuum to continue the bakeout process. The gas is heated before flowing into the chamber. This step requires a heater, which adds to the complexity of the test setup. Additionally, the high pressure requires a very high purity of gas to water vapor ratio, because the partial pressure due to the water vapor in the chamber will not go below that of the gas flowing into the chamber. Furthermore, the outgassing rate at the low vacuum is not very great.
Attempts have been made to enhance this procedure by raising and lowering the pressure while sweeping with preheated argon gas. However, all such attempts have been proposed at the lower vacuums that require a greater gas purity. These attempts may have had a high throughput up to about 6.0.times.10.sup.3 torr.multidot.liter/s, but a low outgassing rate.
It is, therefore, desirable to have a method for performing a bakeout process that can more quickly remove the desorbed gases from a vacuum chamber, and thereby reduce the total time for the bakeout test.